This invention relates to battery powered air sampling pump systems and, more particularly, to a pump system for maintaining constant air flow rate.
U.S. Pat. No. 5,107,713 (the "'713 patent"), assigned to the assignee of the present invention, describes a portable air sampling pump system of the type generally carried or worn by a person in order to collect environmental air samples for determining air quality. The pump system of the '713 patent comprises an air sampling pump having a central processing unit (CPU) for developing pulse width modulation (PWM) control signals for application to a PWM motor driver which is connected to supply electric power to a DC electric motor. The motor is coupled in driving relationship to a diaphragm type air pump and a revolution per minute (RPM) counter is coupled to the motor/pump assembly for providing RPM signals representative of pump speed to the CPU. The CPU includes memory operably associated with it for storing data indicative of predetermined relationships between RPM, PWM and air flow. The CPU is responsive to a commanded air flow signal for generating a corresponding PWM signal for energizing the motor at a preselected RPM. The relationship between motor RPM and air flow rate is obtained by a combination of empirical measurement and interpolation. In one form, the pump is initially energized at a preselected PWM value and the pump flow rate and RPM corresponding to that PWM value are recorded. The pump is then energized at another PWM value and the pump flow rate and RPM corresponding to that value are also recorded. These two values enable development of a characteristic curve for the pump. Tables of PWM values relating RPM and flow rate to PWM are computed from the two measured values. When a desired flow rate is set in the CPU, the CPU generates a PWM signal to energize the pump at an RPM corresponding to the desired flow rate. Thereafter, the RPM of the pump is monitored and the PWM value periodically adjusted to maintain the RPM at the selected value.
While the system described in the '713 provides relatively accurate control of air flow rate, it is not as precisely accurate as is desirable when measurement of some air borne contaminants are being monitored at concentrations measured in parts per million. The '713 patent system uses the assumption that air flow rate is directly approximate to pump RPM. However, a more accurate measurement is needed to accommodate variations in the system such as may be caused by clogging of an input filter designed to remove dust or other debris from the air being sampled.